Transmission system



Patented Sep't. 25.1928.

UNITED STATES PATENT OFFICE.

ELIEB V. 631668, WHITE IPLAINB, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OI NEWYOBK, N. Y., A CORPORATION OF NEW YORK.

TBANBIISSION SYSTEM.

Application filed October 1c, 1925. Serial No. cam.

. This invention relates to transmission systems in which a carrier wave is used for transmitting signals of frequencies within the audible range, and it relates particularly to I systems of this kind in which two or more signals of similar frequencies, but differing in phase or intensity are simultaneously translated, transmitted and faithfully re-. produced with their original similarities and differences.

Systems of this type have heretofore been used for the purpose of locating sounds with respect to fixed and known points, as in submarine soundings, direction and ran e finding. They have also been used for the purpose of providing for a realization at the receiving station of the relative locations of various sound sources with respect to one an other at the transmitting station, as in binaural transmission over wires (or by radio) of musical programs and incidental sounds for talking movies.

Such systems have in the past possessed a serious drawback in the necessity of using one transmission channel for each of the different versions of the same sound. The spectra of frequencies for radio and wire transmission are necessarily limited and with the rapid growth of this art, it has been found inadvisable to adopt this method of transmission.

This is true even in case of radio broadcasting of orchestral programs and the like, in which the sense of location of the various instruments has been found to materially assist in the appreciation of the program by the listener.

Another disadvantage of these systems,

when applied to radio systems resides in the fact, that it has so far been necessary to use an antenna with its associated power equipment for each transmission channel, thus necessitating duplication of expensive equip ment and doubling the power required to transmit the signals a given distance. More over, fading of the different. carrier waves to different degree tends to cause a false apparent shift of position of the originating sound sources. 7

In accordance with the present invention the low frequency signals from two sound translating devices are transmitted as components of a single carrier wave. The carrier is modulated by signals fromone device and the upper side band of the resultant wave is selected and impressed upon the transmission circuit. The same carrier is separately modulated by si als from the other device and the lower si e band of the resultant wave 18 selectedand impressed upon the same transmission clrcuit.

An object of the present invention is to reduce the number of frequency bands required for transmission systems of the type described above, so that the frequency spectrum may be economically utilized.

Another ob ect is to minimize duplication of equipment used in such systems.

A further object is to increase the efliciency w th respect to the power output of a transmltting system of that type. 79

Further features relate to the reduction of cross talk between signals transmitted by such a system.

In order to more fully explain the nature of the Invention, its organization and method of operation, it will be described hereinafter as appl ed to a radio broadcasting system for producing bmauraleifects. It is to be understood, however, that the invention is not limlted to such use nor to the specific details involved, since it may find ready application in various other systems for transmission of sounds, as Well as for transmission of light or other physical variations.

The description should be read in conjunc- 85 tron with the attached drawing, which shows diagrammatically one circuit arrangement of a transmitting and a receiving apparatus.

In the diagram are shown two pick-up or translating devices 10 and 11, which may be of the microphonic or other type adapted to broadcasting purposem, These devices may be placed within the field of musical sounds produced, say, by an orchestra, and may be spaced apart in a manner which may be found most suitable for binaural re roduction. The two devices 10 and 11 will t us receive the same sounds from the instruments, but

the phase relation and intensity of the sounds received from the individual instruments will depend upon the location of these with respect to the devices 10 and 11.

The low frequency currents produced by the pick-up device 10 and representing one' version of the musical sounds pass through a band filter 12 and are impressed upon an amplifier circuit 13.

The currents produced by the pick-up de--. vice 11 and representing another version of the musical sounds pass through a band filter 14 and are impressed upon amplifier circuit 15.

The amplifiers 13 and 15 are shown by way of example as including three element vacuum tubes 16 and 17 respectively, the output circuits of which are connected to the respective output circuits of two repeating and modulating tubes 18 and 19, also of the three element vacuum tube type. A low frequency choke coil 20 is included in the common battery' lead for the output circuits of tubes 16 and 18, and a low frequency choke coil 21 1S similarly provided for tubes 17 and 19.

A high frequency oscillator 22 is connected to the input circuits of modulators 18 and 19, for the purpose of supplyin a carrier wave thereto. The oscillator maybe of the electron discharge type, such as described in U. S. Patent No. 1,356,7 63, October 26, 1920 to Hartley.

lVith the circuit arrangement as shown the carrier wave supplied to the modulator 18 will be subject to the usual repeater action, and will at the same time, be modulated in accordance with the low frequency currents present in the output cireuitof amplifier 13.

The carrier wave impressed upon modulator 19 is similarly amplified and modulated in accordance with the low frequency current present in the output circuit of amplifier 15.

For a description of the modulating action here taking place and the part played there in by the impedances 20 and 21-, resort may be had to Patent No. 1,537,941, May 19, 1925 to Heising,'where a similar system is disclosed in Fig. 2.

As is well known, the high frequency waves resulting from the inter-action between the carrier wave and the speech frequency wave in the modulator tube 18 will be composed of components of the frequency C of the carrier and of the sum and difference frequencies C+S and CS,; where S, represents the speech frequency in this branch of the system. Other components may be present, but they will not effect the operation of the system so far as concerns the present invention.

The high frequency wave present in the output circuit of modulator 19 will similarly be found to be composed of the frequencies C, (1+ S and 0-8,, where S represents the speech frequency in this branch of the circuit.

S and S, being of the same order offrequenc-ies the side bands produced by combining them with C will, of course, be of the same width.

These two waves are now impressed upon the band filters 23 and 24 respectively, of which one is designed to pass a frequency band corresponding to the upper side band, or sumfrequency. component of the present carrier wave; while the other is designed to pass a band corresponding to the lower side band, or difference-frequency component thereof. Reference may be had to Patent No. 1,227,113,

May22, 1917 to Campbell, for a description of the design and operation of band filters or selccting network of this type.

Since filter 23 will pass only an upper side band of the modulated wave, and filter 24 will pass only a lower side hand, there will be present in the common output circuit 25 of these filters the side bands G+S and (3-5,, and due to the somewhat rounded characteristic at the cut off points of both networks, the bands passed by them will be wide enough to overlap at the carrier frequency, so that enough energy of the carrier C will also be present in circuit '25. Thus the resultant output wave will comprise the above mentioned side bands and an unmodulated component of carrier frequency which serves for detecting purposes at the receiving station.

The purpose of the band filters 12 and 14 in the low frequency or pick up circuits in this embodiment of the invention is to prevent excessive cross talk in the receiving system due to this overlapping of the bands passed by the selecting networks 23 and 24. The low frequency filters 12 and 14 are adapted to cut off as much of the lower frequency of the musical notes or voice, received by pick up devices 10 and 11, respectively, as would be consistent with the desired quality of reproduction, and may, therefore, be designed to pass frequencies between 200 and 5000 cycles.

The side bands impressed upon the filters 23 and 24 will thus have adjacent frequency limits of approximately C+200 and C20O cycles, which will permit overlapping at the carrier frequency C, and yet be efi'ective in eliminating cross talk in the receiving circuits.

It appears to be desirable, in a system of the type described above, to take precautions to reduce cross talk because of the high frequency commonly used for the carrier in present day broadcasting systems and the consequent comparative lack of sharpness at the cut off points which would be found in band filters, such as 23 and 24. However, due to the similarity of the two signals trans mitted, a certain amount of cross talk may very well be permitted.

In wire carrier systems, where the carrier frequency is comparatively low, the band filters, corresponding to 23 and 24 in the drawing, may be designed to cut off very sharply at their limiting frequencies, so that the side bands formed in a manner similar to that described above, will not be likely to overlap. The low frequency filters, corresponding to 12 and 14 above, may therefore in such systems be entirely dispensed with, without giving rise to detrimental cross talk at the receiving station. Energy ofthe unmodulated carrier frequency may in this case be applied directly to the'output circuit corresponding to 25 in the drawing.

Returning now to the system described lam above the out ut circuit is connected through ampli cation equipment 26 to the transmision orantenna circuit 27-28.

The wave transmitted from the antenna 27 consists of an upper side band, a-lower side band, and a carrier component. This wave occupies the same width as a single broadcasting channel, but contains the two versions of the original sound. Economy in the utilization of the broadcast spectrum may thus be obtained.

It will readily be seen that this system requires only a single amplification equ pment with its power su ply, as well as a single antenna for transmitting two signals, as a consequence of which the first cost of equipment and the cost of maintenance are materially reduced.

At the receiving station the transmitted wave is picked up by the receivin antenna 30 and is impressed upon the ban filters 31 and 32, which are similar to the filters 23 and 24 respectively. Filter 31 passes the side band C+S and a part of the energy of. the carrier frequency C, while filter 32 passes the side band C--S and some energy of carrier frequency C. By means of detector sets 33 and 34 respectively, of any known type, the modulated and unmodulated components are combined to produce the original low frequency waves S and S wh ch are respectively impressed upon the indlvldual receiving elements 35 and 36 of any. convenient translating device, such as a headset 37.-

The phase differences and relative intensities of the original signals having been retained throughout, due to the similarity of equipment in the two branches of the system, a striking realization of the distribution of the various sound sources at the transmittin station is conveyed to the listener throng his binaural sense.

It should be understood that many other arrangements may be-devised for the broadcasting of binaural progpams, to which this invention may be applied. However, the particular method of modulation, indicated in the drawing, has the advantage that the modulation taking place in the plate circuit of one tube, for example, 18, will have a negligible effect upon the grid circuit of this tube and will therefore not affect the input circuit of the other tube 19, and conversely the modulation effected in tube 19 will not appreciably affect the input circuit of tube 18. The provision of two separate circuits for connecting oscillator 22 to the respective input circuitsv of tubes 18 and 19 will still further assist in the successful separation of the signal S and S What is claimed is:

1. In a carrier system, a method of transmitting signals, which comprises deriving from a single source signal impulses differing in phase from each other, separately modulating a carrier wave by said difierently phased impulses, selecting one side band component of each modulation, and transmitting the selected components over a single channel. In a carrier system, a method of transmitting s1 als which comprises deriving from a sing e source signal impulses differing from each other, separately modulating a carrier wave by the respective derived 1mpulses, selecting the sum frequenc com onent of one modulation, selecting t e di erence frequency component of the other modulatlon and transmitting the selected components over a single channel.

3. In a carrier sgstem, the method of producing blnaural e ects, which comprises der1v1n from a single source signal current impu ses differently phased with respect to each other, separately modulating a carrier wave by said de-phased currents, selecting the upper side band component of one modulation, selecting the lower side band component of the other modulation, simultaneously transmitting the selected side bands over a single channel, selecting and detect- 1ng said components to produce the de-phased currents, and using said de-phased currents to separately reproduce signals.

1. In a carrier system, a method of transmitting signals which comprises receiving two blnaurally related versions of an audible signal, and transmitting over a single channel a carrier wave modulated with waves within the same frequency range in accordance with said versions.

5. In a carrier system, the method of transmlttlng signals, which comprises separately modulating a carrier wave by current inipulses derived from two translating devices, selecting one side band of one modulation, selecting one side band of the other modula-. tion, simultaneously transmitting the selected side bands over a single channel, selecting and detecting said side bands to independently produce said current impulses.

6, carrier system comprising means for deriv ng from a single source signal impulses diiferlng in phase from each other, means for separately modulating a carrier wave by said differently phased impulses, means for selectlng alternate side band components from said modulations, and means for transmitting the selected components over a single channel.

7. A carrier system comprising means for gun deriving from a single source signal impulses differing from each other, means for separately modulating a carrier wave by the respective derived impulses, means for selectmg the sum frequency component of one modulation, means for selecting the difference frequency component of the other modulation, and means for transmitting the selected components over a. single channel.

' 8. A carrier system for producing binaural effects comprising means for deriving from 9. A carrier system comprising two trans-- lating devices spacedin binaural relation, and

means for transmitting over a single channel a carrier wave modulated with waves within the same frequency range in accordance with signals produced simultaneously by said devices.

10. A carrier system comprising two translating devices, means for separately modulating a carrier wave by signals derived from said translating devices, means for selecting the upper side band components of oneinodu lation, means for selecting the lower side band components of the other modulation, means for simultaneously transmitting the selected side bands over a single channel, and means for selecting and detecting said side bands to independently produce said current impulses.

11. In combination a source of carrier energy, means for modulatingsaid carrier energy with waves located withinthe audio fre-- quency range to produce an upper and alower side band thereof, the respective side bands representing the same sound differing in phase by an audibly perceptible amount, and means for simultaneously transmitting said two side bands over a single channel.

12. An oscillator producin a carrier wave,

. two three-element vacuum tu e repeaters havof binaural reception of signals, which comprises receiving in a single path a complex wave composed of a carrier wave with two side bands respectively produced by modulation of a single carrier wave with different waves within the same frequency range and representing sound having binaural characteristics, impressing said wave on separate paths selective of specific components thereof, and detecting the selected components to yield currents adapted to reproduce sound waves having the same bina-ural characteristics as the original sound.

In witness whereof, I hereunto subscribe my name this 7th day of October A. D., 1925.

. ELMER V. GRIGGS. 

